Peer Certificate rejected by chain verifier

I have been facing an issue something like this :Peer Certificate rejected by chain verifier.

the scenario is sftp(sender channel ) ->PI->Thirdparty system.In the receiver channel sftp private key needs to be used but ,the keystore is not getting populate in sftp receiver channel .Attached is the documentation of the issue in detail .we have also tried several sap notes :2038612,PI version is 7.4 ,is it anything related to issue of support pack PI .